Stable Isotope Effects of Biogas Slurry Applied as an Organic Fertilizer to Rice, Straw, and Soil

Biogas slurry (BS) is now increasingly used for organic rice production in China. However, the isotopic response and fractionation of different BS application rates to characterize organic rice cultivation have not yet been investigated. In this study, different fertilizer treatments were applied to rice paddy soil including urea, BS with five different application rates and a control with no fertilizer added. Multiproxy analyses (% C, % N, 5'C-3, delta N-15, 5(2)H, and delta O) of rice, rice straw, and soil were undertaken using elemental analyzer-isotope ratio mass spectrometry. Rice, straw, and soil showed only minor isotopic and elemental variations across all fertilizer treatments except for (5'N. 5'N-5 values of rice and straw became more positive (+6.1 to +11.2 parts per thousand and +6.1 to +12.2 parts per thousand, respectively) with increasing BS application rates and became more negative with urea fertilization (+2.8 and +3.0%c, respectively). The soil had more positive delta N-1 values after BS application but showed no significant change with different application rates. No obvious delta N-1 isotopic differences were found between the control soil and soils fertilized with urea. N fractionation was observed between rice, straw, and soil (Delta(rice-soil) -2.0 to +4.3 parts per thousand,Delta(straw-soil)-1.9 to +5.3 parts per thousand) and their isotopic values were strongly correlated to each other (r > 0.94, p < 0.01). Results showed that % C, % N, delta C-15, 8(2)H, and 6180 in rice displayed only minor variations for different fertilizers. However, delta C-18 values increased in response to BS application, confirming that BS leaves an enriched N-15 isotopic marker in soil, straw, and rice, indicating its organically cultivated status. Results from this study will enhance the stable isotope 8'N databank for assessing organic practices using different fertilizer sources.

Automated summary

This study investigates the isotopic response and fractionation of different biogas slurry (BS) application rates on organic rice cultivation in China. Results show that increasing BS application rates lead to enriched N-15 isotopic markers in soil, straw, and rice, indicating its organically cultivated status. N fractionation was observed between rice, straw, and soil, with their isotopic values showing strong correlations.